The smartest move for Ron Paul is to find a general or admiral who backs his foreign policy. This is the pre-eminent area where Obama can outmanouver him and appear as a "centrist" or where a third party rebellion can rise up among neo-cons on hawkism. It also gives disciplined credibility to the ticket. Maybe Admiral William Fallon?
Alternatively, although less seductive electorally, Gary Johnson would be an effective pick. Works to more aggressively target liberals and progressives, also targets New Mexico as a battleground state, and gives an executive's experience to the ticket.